The invention is a diversity antenna arrangement. Diversity antenna systems are used for receiving frequency-modulated signals preferably in the metric waveband.
An antenna diversity system destined to yield substantial improvements in signal quality for FM radio reception on motor vehicles requires at least two antennas.
On account of engineering and design aspects, it is preferable that these antennas be shaped to integrate into the body of the motor vehicle. This is best achieved by using one of the vehicle's windows as the antenna location. Besides the windshield, which is in many cases the most unfavourable antenna location due to the serious amount of electromagnetic interference caused by the engine, the rear window is most suitable for carrying antennas.
When developing rear-window antennas, the rear-window heater structure has a shape and dimensions given by the engineering and design aspects specific to each type of vehicle, which should be thoroughly investigated.
A dual-antenna structure on the rear window for use with diversity systems is proposed in EP 0065263. This arrangement consists of a primary antenna placed in a section of the window that is free of heater wires and a secondary antenna the signal of which is tapped in a suitable way at the lowermost wire of the heater structure.
However, such a configuration has several drawbacks: it is not suitable for use with windows where the entire glass surface is covered by the heater structure; the additional amount of air pressure introduced by the primary antenna is undesirable due to specific design constraints; the receiving performance of the primary antenna with FM radio signals on circularly or vertically waves is unsatisfactory, as the vertical dimension of the area not covered by the heater structure is insufficient.
A dual-antenna configuration on a single window is also proposed in DE 3220279. This arrangement features multiple antenna signal taps with small intermediate spacing.
If used with a diversity system, such an arrangement yields good results in regions with minor differences in the propagation delays of the wave portions involved and low resulting multipath distortion. In regions where propagation is less straightforward and where substantial differences in propagation delay exist (e.g. mountainous areas), however, such an arrangement has major disadvantages compared with two single antennas with a larger spacing between their tap points. For the above-mentioned reasons, the receiving performance of a configuration according to DE 3220279 is also unsatisfactory for circularly or vertically polarized waves.